I went to the local craft store and picked up a texture medium called "Black Lava Texture Gel". It's in the art section, used to texture acrylic paint. It's made by Liquitex, and it's a little pricey, but it made a suitably lumpy asphalt texture when mixed with the right shade of dark gray...
Back when I was car modeling, I used sandpaper to simulate a gravel driveway...it should do for asphalt, too.